The present invention relates to a method of determining the duty ratio for operating a solenoid for use in a duty-ratio-controlled solenoid valve.
Many various electrically controlled devices employ a solenoid for converting an electric control signal into controlled movement. More specifically, the operation of a solenoid-controlled device is controlled by selective energization and de-energization of the solenoid. More accurate control in smaller increments requires a solenoid to be energized and de-energized by controlling the duty ratio of the solenoid.
As disclosed in a Japanese Patent Application filed by applicant, for example, the operation of a servo unit for controlling the speed reduction ratio and the clutch of a hydraulic continuously variable speed transmission is controlled by a pair of duty-ratio-controlled solenoid valves. According to the disclosed control process, a time interval for which a solenoid is to be operated is detemined within a basic period (e.g., 10 milliseconds) in order to obtain a desired duty ratio in each basic period, and the solenoid is operated only for the determined time interval in each basic period.
Since a control signal applied to the solenoid of a solenoid valve is an electric signal, the time interval of operation of the solenoid can freely be set to a considerably small value. The output operation of the solenoid however, is in the form of mechanical movement, and the solenoid valve actuated by the solenoid effects mechanical movement. Therefore, there is a certain limitation on the minimum controllable time for which the solenoid valve can be operated by the solenoid, and the time interval of operation of the solenoid within the basic period cannot be reduced below that limitation, with the result that the duty ratio of operation of the solenoid cannot be set to an excessively small value. For example, if the shortest possible time period of operation of a solenoid valve is 1 millisecond within each basic period of 10 milliseconds, then the minimum duty ratio of the solenoid is 10%.
While a motor vehicle equipped with a hydraulic continuously variable speed transmission as described above is running under a normal condition, only a very small amount of working oil is supplied to and discharged from a reduction ratio controlling servo cylinder, and hence the duty ratio for controlling the operation of the solenoid valves is required to be very small. Heretofore, however, it has been difficult to attain a desirable small duty ratio for the reasons described above, and the solenoid valves could not be accurately controlled.
According to one conventional method, a smaller duty ratio can be obtained by increasing a basic period. In as much as the duty ratio is calculated per basic period, the interval in which the duty ratio is calculated is also increased, and the response of the control system to an abrupt change to be made in the duty ratio is lowered. The control system response may be prevented from being lowered by employing an interrupt process. However, the interrupt process itself requires its own control, making the overall control complex.